Railcars are presently essentially built of two wheel units of four wheels each carrying a main car body. Each wheel unit comprises two axles with two wheels each fixedly connected to the axle. These axles are arranged with relatively small distance from each other in a base element or frame forming a wheel unit or running gear. The main body of the railcar is carried by two such wheel units.
The wheels are frequently built to have a slightly tapered roll surface, which tapers towards the outside of the unit. In other words, the tapered roll surface of each wheel has a larger diameter at the inside than at the outside of the wheel. The insides of the two wheels on one axle are facing each other, whereas the outsides of the two wheels on one axle are facing away from each other.
During the operation of such railcars, an oscillating or "hunting" is observed. This hunting is a periodic turning of the wheel units that can result into a dancing of the railcar, particularly when the railcar is empty and the car moves at a normal traveling speed of around 40 to 60 miles per hour. This dangerous oscillating causes the railcars to ride unevenly. Furthermore, the oscillating causes high maintenance costs for rails and railcars, see particularly "Railway Age", Nov. 24, 1975, Vol. 176, No. 22, page 40, center column.